1. Field of the Invention
This invention relates to a semiconductor memory device.
2. Related Art
In development and mass production of memories, the signal difference between memory cells storing data “0” (hereinafter, “0 cells”) and memory cells storing data “1” (hereinafter, “1 cells”) is an important parameter. For example, a reference voltage required for detecting data in memory cells is set to the intermediate potential between the potential of the data “0” and the potential of the data “1”. At this time, it is assumed that the detected voltage for 0 cells is lower than the one for 1 cells. In this case, the reference voltage is set based on the maximum value in voltage distribution of 0 cells' data and the minimum value in voltage distribution of 1 cells' data. Accordingly, the maximum voltage for 0 cells' data and the minimum voltage for 1 cells' data are also important parameters.
To measure the maximum voltage for 0 cells' data and the minimum voltage for 1 cells' data, there has been the following measurement process for conventional memories. A certain voltage is set as a reference voltage and data detection (read) is performed for all cells while changing their addresses to output the resultant data. The reference voltage is then changed by a step voltage, and the data detection and output are repeated. Based on the data outputted from output circuits, the voltage distribution of the data “0” and the voltage distribution of the data “1” are obtained. The maximum value for 0 cell and the minimum value for 1 cell are obtained from these voltage distributions and the reference voltage is determined based on the maximum value and the minimum value. Such processes for conventional memories require a considerably long time to determine the reference voltage.